1. Field of the Invention
This invention resides in the technologies associated with solutions and suspensions of sulfur and sulfur-containing compounds, with particular interest to methods of transport of sulfur by pipeline and other vessels where the deposition of solids is sought to be avoided, eliminated, or removed, or where solids are purposely deposited. Among the many areas of application of this invention are chemical processes that produce elemental sulfur as a product, chemical processes and products in which sulfur is used in combination with ammonia or other nitrogen compounds, and in the recovery of both useful hydrogen values and sulfur values from chemical processes for the abatement of hydrogen sulfide, including those in which sulfur dioxide is or may be made as a by-product, co-product, intermediate product, or waste product of hydrogen production.
2. Description of the Prior Art
Western Canada and the United States each produce approximately 1×107 metric tons of elemental sulfur each year, primarily as a by-product of natural gas production and petroleum refining, and with the advent of NAFTA, Mexico stands to contribute comparable amounts through its natural gas production and native sulfur mining industries. Sulfur is also produced as a by-product in petroleum refinery operations, coal-fired power plant operations, and tar sands development, and in any industrial process that reduces the sulfur level in fuels or effluents for purposes of complying with air quality standards.
While industrial chemicals and commodities can be transported long distances by pipeline, in many cases more economically than by rail or other forms of shipment, pipeline transfer has not been used for sulfur or for only short distances at most. This is due to the high melting point of sulfur, the corrosiveness of sulfur when dissolved in typical solvents or when in contact with air or moisture, and the tendency of sulfur to precipitate from solution. When shipped as a solution or slurry, sulfur tends to deposit on the pipeline walls, resulting in plating, plugging, and line blocking, all of which lead to unreliability, high maintenance, and excessive power consumption.
Pipeline systems, like energetic systems in general, lose heat to the environment by radiation. Thus, in temperate zones under normal ambient conditions, the frictional heat and impulse power flow generated over time from the pipeline system combined with the heat loss that occurs at the exterior surface due to radiation cause the mass at the interior of the pipeline to be warmer than the pipeline itself. This causes the pipeline wall to be cooler, or permits it to be held at a cooler temperature, than the interior mass of moving product. As those skilled in the art are aware, the relation between solute and solvent in a solution is not static but instead one of dynamic equilibrium due to continual precipitation and re-dissolving under stable conditions. The solubility of a solute in most hydrocarbon solvents or in water or aqueous media declines markedly with decreasing temperature. As a result, the solute forms deposits over time, with more deposition occurring in cooler regions of the fluidic mass. This is indeed true in the case of sulfur, which has been found to precipitate faster than it dissolves in regions that are proximate to normally cooler heat transfer surfaces such as pipeline walls, flanges, fittings, and joints. The resulting deposits will plug the flow passages unless energy is supplied that will keep the fluidic mass moving fast enough to prevent plugs from forming. Methods for heating pipelines are complex and expensive.
Until recently, much of the sulfur produced in the United States was obtained by the mining of native sulfur reserves, particularly those of the Gulf Coast, using the high energy-consuming Frasch process. High energy prices have since caused curtailment or abandonment of many Frasch operations and many large mineral reserves of native elemental sulfur remain undeveloped.
The storage and disposal of sulfur pose challenges as well, particularly those arising from environmental concerns. Disposal in an environmentally sound yet economical manner is difficult to achieve. Disposal currently consists of converting molten sulfur to solid blocks for above-ground storage, injecting sulfur as H2S into geologic formations, or oxidizing hydrogen sulfide to sulfur oxides and injecting the sulfur oxides underground for storage.
Sulfur is primarily used in the production of sulfuric acid which is then used for producing phosphoric acid and phosphate derivatives at locations near large mineral deposits of phosphate rock. These locations are found primarily in Australia, Brazil, Florida, Idaho, the Middle East, and North Africa. Phosphate operations are typically very distant from sulfur production facilities, and many phosphate operations have been curtailed or shut down due to high energy prices or to supply disruptions caused by a lack of new power generating capacity despite increasing demand. This is particularly true in the western United States.
In the Middle East where the cost of power is extremely low, sulfur is conveyed through a long pipeline that is electrically traced to keep the sulfur at an elevated temperature and to facilitate re-starting of the flow when the pipeline becomes clogged due to sulfur solidification during upset conditions. Rail transport is used in Alberta, Canada, for shipping dry sulfur by unit trains to ports on the Pacific coast, and for shipping molten sulfur, which is susceptible to premature solidification, to points east and south. Shipping by unit train requires multiple locomotives, high-performance rail cars, and heavy-duty trackage, and is inherently inefficient due to the need to return empty rail cars to the sulfur source. When molten sulfur is shipped long distances, the tank cars must be steamed once they reach their destinations so that any solidified sulfur can be re-melted before the sulfur is off-loaded.
Of further potential relevance to this invention is the prior art relating to ammonia production. Ammonia plants are often located near natural gas reserves where sulfur is produced as a by-product. Anhydrous ammonia is conveyed by many modes of transportation including modified tank ships, barges, pipeline, rail, and truck, and large amounts of ammonia are imported from various parts of the world. A major proportion of the ammonia production capacity in North America is currently shut down due to high the cost of natural gas as a raw material and to low product prices.
The properties of mixtures of sulfur and anhydrous ammonia are reported by Ruff, O., and Hecht, L., in “Concerning Sulfammonium and Its Relation to Sulfur Nitride (writer's translation),” Zeitschrift für Anorganische Chemie, Vol. 70, p. 49-69, Leopold Voss, Leipzig, 1911, and by Ruff and Geisel, E., published under a similar title in Berichte der Deutschen Chemische Gesellschaft, Verlag Chemie, Berlin, v. 38, p. 2659, 1905. In these disclosures, Ruff et al. teach that sulfur and liquid anhydrous ammonia react to form sulfur nitride in accordance with the reaction:10S+4NH36H2S+N4S4 
This reaction is a recognized synthetic route to sulfur nitride. While stable in air, the nitride (which is also referred to as “nitrogen sulfide” in Chemical Abstracts) is an explosive that converts to the elements in a violent reaction if subjected to shock or rapid heating under certain conditions. Because of this explosive nature, few if any investigations of non-polymeric sulfur nitride have been reported.
Of still further potential relevance to this invention is the state of the art of sulfur dioxide. Sulfur dioxide is in large demand as a raw material for the manufacture of sulfuric acid, but a limiting factor is the high expense of transporting sulfur dioxide, as explained in the monograph by Rieber, M., Smelter Emissions Controls: The Impact on Mining and the Market for Acid, prepared for U.S. Department of the Interior Bureau of Mines, March, 1982, as quoted in U.S. Congress, Office of Technology Assessment, Copper: Technology and Competitiveness OTA-E-367 (Washington, D.C.: U.S. Government printing Office, September 1988, page 165, Box 8-A): “Liquid SO2 has a very limited demand in the United States, but, owing to its relatively high price per unit weight, it can be shipped long distances. It is still extremely expensive to transport, however, because it requires special pressurized tank cars that usually return empty. The market is too small to justify cost saving measures such as unit trains or special ocean tankers.”
Of still further potential relevance to this invention is the state of the art of natural gas production from natural gas reserves having high hydrogen sulfide content. Fouling and impairment of wells and pipelines by premature sulfur deposits is common and results in expensive maintenance problems and capacity losses due to shut-downs and extended off-line periods required for inspection, cleaning or replacement.
Of still further potential relevance to this invention is the state of the art of hydrogen sulfide and the recovery from hydrogen sulfide of both sulfur values and hydrogen values. Hydrogen sulfide is produced as a by-product of natural gas production, and also as a by-product of refinery operations and many processes that are intended to remove sulfur from fuels. Canada and the United States each produce about 1×107 metric tons of hydrogen sulfide per year. Because of its extreme acute toxicity, flammability, noxious odor, insidious odor sensory depression, and corrosiveness, almost all hydrogen sulfide is converted to elemental sulfur and water at or near the site where the hydrogen sulfide is produced. Conversion is achieved by the Claus process, in which one mole of hydrogen sulfide is oxidized to water and one mole of sulfur dioxide which is then reacted with two additional moles of hydrogen sulfide to produce elemental sulfur and more wastewater or steam. All the hydrogen value of hydrogen sulfide is thus lost to wastewater and low quality steam. In North America, for example, about 1.2×106 metric tons of hydrogen are lost in this way each year. The economics of hydrogen sulfide are summarized by Zaman and Chakma in “Production of hydrogen and sulfur from hydrogen sulfide,” Fuel Processing Technology 41 (1995), 159-198, Elsevier Science B.V., as follows:                “The diverse attack on hydrogen sulfide to obtain two salable products is very striking. Every year a large amount of potential resource is being wasted and there is no doubt it should be stopped. The success in the development of a suitable technology for the production of hydrogen and sulfur will signify the attainment of the triple objectives of waste minimization, resource utilization, and environmental pollution reduction.”        
Hydrogen is commonly produced by steam reformation and water shift reactions using natural gas (methane) or other carbon-based reductants including petroleum derivatives and coal. Natural gas is in short supply, however, and shortages and high prices are likely to persist for the foreseeable future. Also, for every mole of methane consumed, the process generates a mole of carbon dioxide. Thus, for example, in the production of ammonia from hydrogen and nitrogen, about a million tons of carbon dioxide are produced (on a stoichiometric basis) for every million tons of ammonia made, in addition to the carbon dioxide produced by combustion to provide energy for other process needs. Some of the carbon dioxide can be consumed as a raw material to make urea, but the economic value that is gained from the use of carbon dioxide in this manner is insufficient to compensate economically for the loss of methane, since CO2 is readily available from other sources in ways that do not involve methane consumption. Also, as those who are familiar with the proposals for a hydrogen economy are aware, the use of hydrogen will continue to grow and thereby exacerbate the serious problem of excess CO2 emissions, a recognized contributor to global warming. This is because large-scale hydrogen production uses carbon-based reductants and will continue to do so for the foreseeable future despite significant advances in renewable energy technologies. Thus, in the production of hydrogen by methane reformation under the best circumstances, at least five and one half tons of carbon dioxide will be produced for each ton of hydrogen. This could be alleviated by producing hydrogen from non-carbon sources in a process whose by-product is a solid mineral such as gypsum rather than greenhouse gases.
Known schemes for producing hydrogen from hydrogen sulfide are as follows:H2S+COCOS+H2  Reaction 1H2S+NONOS+H2  Reaction 2H2S½S2+H2  Reaction 3
Reaction 1 is the subject of U.S. Pat. No. 4,618,723 (Herrington et al., Oct. 21, 1986)), while both Reactions 1 and 2 are discussed in U.S. Pat. No. 3,856,925 (Kodera et al., Dec. 24, 1974). The most striking recent development pertaining to Reaction 1 is work supported in part by the National Science Foundation and assigned to Lehigh University, disclosed in U.S. Pat. No. 6,497,855 (Wachs, Dec. 24, 2002). The Wachs patent teaches that an internal stream of COS can be catalytically oxidized with O2 to yield SO2 per the reactionCOS+O2→CO+SO2  Reaction 4while regenerating and recycling an internal stream of CO that is used to generate more hydrogen from fresh hydrogen sulfide feed according to Reaction 1 above. The overall reaction is as follows:H2S+O2→H2+SO2  Reaction 5
The practicalities of this scheme are constrained by the burden of SO2 disposition. The Wachs disclosure offers two alternatives for SO2 disposal: 1) use in the production of sulfuric acid, and 2) recycle of SO2 for reduction by two additional moles of hydrogen sulfide to produce elemental sulfur and water as for example by the Claus process. The former is thought to be an attractive choice at sites near large-scale consumers of sulfuric acid such as petroleum refineries. Unfortunately, however, the inefficiencies and high transportation costs of sulfuric acid make this impractical at remote hydrogen sulfide sources such as the Wyoming or Alberta sour gas fields. Large-scale remote sulfuric acid production would also have to compete with sulfuric acid produced as a smelter by-product. The disposition of surplus acid at remote locations raises environmental concerns, since acidulation of carbonaceous ore bodies such as limestone presents a variety of problems including excess CO2 emissions.
Disposition of the SO2 as recycle through the Claus process limits the yield of hydrogen from hydrogen sulfide to about one-third at best, based on the overall stoichiometry (combining Claus with Reaction 5) as follows:3H2S+O2→H2+2H2O+⅜S8  Reaction 6
The scheme of Reaction 6 also raises economic and environmental concerns due to its production of surplus elemental sulfur at remote locations. This reduces the amount of economic value that can be extracted from the hydrogen sulfide.